# Board-backed card mailers, C6, 100 pack > Stiff envelopes that keep cards flat in the post. - Mailers, Best value - Typical UK price: £10 to £18 (a typical band, not a live price) - Platforms: Whatnot, eBay, Vinted, TikTok Shop, Depop, Etsy - Orders a week: 0–2000 - Not yet checked by a person A sleeved and top-loaded card in a rigid mailer stays flat through sorting machines and usually ships as a Large Letter, which costs far less than a parcel. ## Kit rankings - #4 in [Best mailing bags for Vinted](https://sellersetups.com/kit/mailers-vinted) - #3 in [Best mailing bags for Vinted and card mailers](https://sellersetups.com/kit/mailers) ## Setups - [Pro Whatnot streaming setup for card breaks](https://sellersetups.com/setups/pro-whatnot-streaming): A two-camera studio with app-controlled lights, a button pad for angles and a packing line, for card breakers posting 80 to 300 orders a week. - [Whatnot card break setup for sports and football cards](https://sellersetups.com/setups/sports-card-breaks): An overhead rip camera, a sorting system for hits per buyer and Large Letter packing, for running football and sports card breaks on Whatnot and eBay. - [Whatnot card setup: £300 starter kit](https://sellersetups.com/setups/whatnot-card-starter): Camera, lights and packing kit for running your first Whatnot card shows from a desk and shipping the orders next morning, for about £300. ## Who should keep C6 board-backed mailers in stock Card sellers posting one to a handful of toploaded singles use these, and so does anyone sending photos, prints or postcards. Vinted, eBay and Whatnot buyers all expect a card to arrive flat. A 100 pack at £10 to £18 covers a busy month for a breaker and a few months at a casual pace. If most of your orders are lots of 50 cards or more, a small box serves you better. ## Inner size, board and flap on a C6 mailer C6 is the ISO paper size of 114 by 162 mm, and a C6 envelope fits a 3 by 4 inch toploader with room to spare. Read the listed inner size, as some board-backed mailers are cut slightly smaller than true C6. Choose a full board back and a board front, or at least a stiff board back with a printed "Please do not bend" line. A peel-and-seal flap saves you reaching for tape on every order. The finished mailer has to fit within Royal Mail's Large Letter limits for thickness and weight to go at that price. A toploader plus a team bag sits well inside most thickness limits, but double stacks may not; Royal Mail's current size guide has the figures. ## Packing a card order in under a minute Sleeve the card, load it into a toploader, seal the toploader in a team bag, then slide the bundle into the mailer. For two or three cards, lay the toploaders side by side, never stacked loose, and tape them together with painter's tape. Stick the label on the front, away from the flap, and weigh the finished envelope on a pocket scale before you buy postage. A 500 g precision pocket scale reads light card orders to the gram. ## Padded envelopes, boxes and poly bags for cards Kraft padded envelopes, D/1, 100 pack, are the fallback. They cushion bumps, but the bubble lining flexes, so add a piece of board for any toploaded card. Small single-wall boxes, 25 pack, give bigger lots crush protection an envelope cannot match. Switch to a box once a card order outgrows the Large Letter limits. Grey mailing bags, 10 x 14 in, 100 pack, suit clothing but leave cards free to bend, so keep them away from your card stock. ## Mailer cost against a £3 card sale At £10 to £18 per 100, the mailer adds about 10p to 18p per order. With a sleeve, toploader and team bag, a full card pack still comes in well under 50p before postage. That leaves room on a £3 to £5 card sale. Track your costs per order for a month to see whether a bigger pack brings the mailer price down enough to pay off. ## Common mistakes - Toploader every card before it goes in. A bare card in a mailer bends at the corners. - Two toploaders stacked on top of each other can push the envelope past Large Letter thickness; lay them side by side or check Royal Mail's current size guide. - A label over the flap means the buyer tears through it. Place it on the plain front. - Guessed weights risk underpaid postage and a fee for the buyer, so weigh each finished mailer on a scale. - Mailers cut smaller than true C6 jam toploaders in tight; check the inner dimensions before you order a big pack. ## Guides - [How to run football card breaks on Whatnot](https://sellersetups.com/guides/football-card-breaks): Running Premier League and Champions League card breaks on Whatnot: picking a format, filling spots, ripping on camera and posting each buyer’s cards. - [How to post Pokémon cards in the UK](https://sellersetups.com/guides/how-to-post-pokemon-cards): Sleeve, toploader, team bag and a board-backed mailer: the layers that get a Pokémon single to a UK buyer flat, plus when a Large Letter stops being enough. - [What Whatnot, Vinted and eBay sellers keep](https://sellersetups.com/guides/reseller-earnings-maths): UK fees on Whatnot, Vinted and eBay, worked examples of profit per item after postage and packaging, and the levers that change what you take home. - [How to sell Pokémon cards on Whatnot: setup and kit](https://sellersetups.com/guides/sell-pokemon-cards-live): The camera, light, sound and card protection a Pokémon seller needs for a first live show, and the order to buy it in. - [How to start a Pokémon card business in the UK](https://sellersetups.com/guides/start-pokemon-card-business): Turning a Pokémon card collection into a UK side business: choosing what to sell, where to sell it, stock records and when HMRC treats it as trading. ## Questions ### Does a toploaded card in a C6 mailer go as a Large Letter? Often it does, if the finished envelope meets Royal Mail's Large Letter size, thickness and weight limits. Confirm against Royal Mail's current size guide before you rely on it. ### How many toploaders fit in one C6 mailer? Two side by side fit in most true C6 mailers. Stacking more raises the thickness, so measure the result. ### Do I need a padded envelope instead? Not for single cards. Bends do more damage to a card than knocks, and the board stops bends. Use padding for odd-shaped items. ### Can I write "Do not bend" on a plain envelope instead? You can, but sorting machines ignore it. The board keeps the card flat. ### Will a C6 mailer fit a graded slab? No. A slab is too thick and too long for C6. Post slabs in a small box with padding.